This error (from the FLEXnet licensing system) seems to occur rarely, but for no very clear reason.

First advice: simply close all instances of Excel and try running the software again. Usually the error does not repeat, which is one reason it's so difficult to pin down.

If the error does repeat, run the software with administrative rights. This resolved it for both users who reported this experience. One user right-clicked the @RISK icon and selected Run As Administrator; the other logged in to Windows with an administrator account. If running as administrator succeeds, you may want to try running as a regular user after that. (Our end-user software does not normally require administrative rights to run; a Windows "regular user" account should be fine. But privileges and access control are more complex in every new version of Windows, and it's possible to create an account that shows as regular user in Control Panel even though it does not have the normal permissions of a regular user. In that case, running as administrator may be a quicker solution than trying to untangle the details of the permissions.)